Amniote origins

Prior to the appearance of amniotes, all tetrapods reproduced in the water. With most extant amphibians, fertilization takes place externally; the same can be assumed for the earliest tetrapods. Typically, when two amphibians mate underwater, the male discharges sperm onto mature eggs that are being released from the body of the female. The egg mass is covered in a gelatinous substance; this sticky substance adheres the eggs to a surface such as a rock. Soon, an aquatic larva hatches from each egg. It swims freely, breathes through gills, eats food, and grows rapidly. The larva undergoes

An amniotic egg protects the embryo within the sealed environment of an egg that can be laid out of the water. The egg is also a self--sufficient environment, needing only oxygen from the outside.

metamorphosis during which it grows limbs, develops lungs, and leaves the water as an air-breathing creature.

Reptiles and later tetrapods are considered more advanced than amphibians partly because of their reproductive strategy. This reproductive strategy is at the heart of being a reptile, a mammal, or a bird—a commonality that unites these vertebrates into a group known as the Amniota. An amniote protects the embryo of its offspring within the sealed environment of an amniotic egg. The egg is more or less impervious and protects the embryo from the outside environment. The egg is also a self-sufficient environment, needing only oxygen from the outside. The term amniote is derived from the word amnion, the name for a membrane that forms a fluid-filled sac around the embryo in the eggs of reptiles, mammals, and birds.

All extant amniotes are derived from the first such animals, the earliest reptiles, that arose during the Late Carboniferous Period. The early reptiles, like most of their living descendants, including birds, evolved a shelled egg that could be left exposed to the air in a dry environment. Amniotes also include the mammals and some kinds of reptiles that give birth to live young but retain the same general pattern of egg and embryo development inside the body of the mother.

The first reptiles succeeded by breeding out of the water, exploring new habitats and extending their ecological and geographic range. Just how the first egg-laying reptiles evolved is a paleonto-logical puzzle with little hard evidence but an extremely sensible theory. Alfred Romer once summed up the entire tale by concluding that, "It was the egg that came ashore first before the adult was ready to follow it." This statement at first seems to be disconnected from reality, as it seems to imply that reptiles would lay eggs on land without being ready to live on land. In view of the fact that many of the tetrapods of the Middle Carboniferous were still semiaquatic, one of the last vestiges of their water-bound life would certainly have been their practice of breeding in the water. The behavior associated with reproduction, however, is not entirely driven by an organism's genetic signals. Reproduction is also strongly influenced by environmental factors. Those factors can favor an organism with a slightly different reproductive strategy that gives its offspring a better chance for survival. Romer speculated that the lakes and streams of the Carboniferous were teeming with fishes, arthropods, and aquatic tetrapods, all anxiously tracking down their next meal. This belief is supported by the rich fossil record of aquatic life from the time. Even today, amphibian eggs, once hatched into tadpoles, provide an abundant food source for other aquatic creatures. To compensate for the guaranteed loss of most of their young, amphibians lay an enormous number of eggs to increase the odds of continuing their kind. It is not unusual for a species of frog or toad to lay several hundred eggs in a single mass. The northern leopard frog and eastern American toad vastly exceed the norm by laying between 4,000 and 8,000 eggs at one time.

It would appear that some early tetrapods took the protective step of laying their eggs on land. Just how a gelatinous egg mass could survive out of the water does not require too much imagination.

In a rain-forest habitat such as that seen in many Carboniferous terrestrial ecosystems, there would have been many damp, moist, continually wet places under logs, beneath ground-covering plants, in tree stumps, and in other shaded places where amphibians could deposit their eggs and breed. In fact, there are many species of frogs and salamanders today that do just that. Laying jelly-covered eggs in a moist, protected spot on land may have been the first step in the evolution of amniotes. Over time, the designs of eggs laid on land were selected to favor those that began to develop special membranes, including a hard outer shell that protected the embryo but also permitted gas exchange to let oxygen inside.

The transition from breeding in the water to laying eggs on land necessitated leaving behind the waterborne tadpole, the larval stage of growth. How this transition was made is unknown from the fossil record. An intermediate step toward fully terrestrial hatch-lings might have resembled the manner in which several species of frogs care for their larval young today. Instead of letting their tadpoles swim freely in open water, some frogs have developed special pouches on their bodies in which they carry the tadpoles until the young metamorphose into their terrestrial form. Perhaps some early amphibians had a similar practice while making the transition to land, possibly skipping the tadpole stage entirely, with hatchlings born as immature but wholly formed imitations of the adults.

The first reptiles were tiny in comparison to the hulking amphibians that dominated the swamps of the Carboniferous. Their small size is probably related to the development of the amniotic egg. By skipping the tadpole stage, reptiles lost an opportunity to grow larger before becoming acclimated to life outside the water. This possibly explains why the first known reptiles measured only a few inches long: Skipping the tadpole stage made for smaller individuals. Small size was not a trait that would characterize reptiles for long, however. By the Permian Period, as amniotes diversified, so, too, did they grow larger and more dominant in whatever habitat they occupied.